A variety of methods and devices are known for measuring the osmotic pressure exerted by solvent molecules diffusing through a semipermeable membrane. Commercially available devices for measuring the osmolality of solutions via membrane osmometry include the 4400 series colloid osmometers made by WesCor (Logan, Utah) and the type 1.00 Knaur membrane osmometers. Other devices for measuring the osmolality of solutions through vapor pressure osmometry are commercially available. An example of this type of device is the VPO model 070 made by UIC Inc. (Joliet, Ill.). Still another known osmometer device operates by measuring freezing point depression. An example of this type of device is the Precision Systems Inc. (Natick, Mass.) Osmette XL product line. Other known osmometer variants include isopiestic vapor equilibrium osmometers and submerged dialysis bag osmometers. In addition, osmolality has been determined by measuring boiling point elevation. The vapor pressure osmometer, boiling point elevation, freezing point depression, and isopiestic methods measure the oncotic pressure (the osmotic pressure exerted by colloid in the solution) of the solution, that is, the osmolality of the solution including the contribution of low molecular weight components such as salts. The present invention is directed to methods for measuring the osmotic pressure of solutions, excluding the contribution of small molecules.
Membrane osmometers and the dialysis bag techniques measure the solution equilibration across a semi-permeable membrane, thus excluding the direct contribution of small permeable molecules. One example of a membrane osmometer is taught in U.S. Pat. No. 4,150,564, titled "OSMOMETER FOR COLLOID OSMOMETRY," by Wayne K. Barlow, et al., Apr. 24, 1979. The present invention is an improvement upon existing membrane osmometer designs, but relies on the same basic principle of establishing an equilibrium across a semi-permeable membrane. Typical membrane osmometers employ pressure transducer technology to directly measure the evolved osmotic pressure difference between a reference cell and the sample solution across a semi-permeable membrane. In the present invention, transducer technology is not required for measuring the osmotic pressure. Typical commercial osmometers are designed to minimize the sample volumes by introducing the sample into a meandering channel above the semi-permeable membrane that maximizes surface contact while minimizing sample volume. This meandering channel geometry is eliminated in the present invention because it limits the usefulness of commercial osmometers to low viscosity, non-fouling solutions and is particularly unsuited for complex solutions (dispersions and emulsions).
Dialysis bag techniques, (Essafi, W. Structure Des Polyelectrolytes Fortement Charges, PhD thesis, Universite Pierre et Marie Curie, Paris, 1996) involve filling a semi-permeable dialysis bag with the sample solutions of unknown osmotic pressure and immersing it in a large volume of solution of known osmotic pressure. The sample changes concentration until an osmotic equilibrium is established. Then the sample is removed and its concentration at that known osmotic pressure is determined using other techniques (spectrophotometrically or gravimetrically). The present invention may be used in a mode of operation similar to this. In this mode of operation the sample solution is allowed to equilibrate to a known imposed air pressure. Then the concentrated sample is removed and the concentration determined separately. However, this method of operation is not the optimal nor is it the preferred embodiment of the method of the present invention.
The prior art also suggests use of polymer solutions of known osmotic pressure to have as reference solutions (rather than solvent) using commercial osmometers or other direct force measurement techniques to allow for measurement of higher osmotic pressure solutions, (Rau, Donald C.; Parsegian, V. Adrian. Direct Measurement Of Temperature-Dependent Solvation Forces Between DNA Double Helixes. Biophys. J. (1992), 61(1), 260-71; Sidorova, Nina Y.; Rau, Donald C. Removing Water From An EcoRI-noncognate DNA Complex With Osmotic Stress. J. Biomol. Struct. Dyn. (1999), 17(1), 19-31). This method works reasonably well for extending the pressure range available on commercial osmometers but does not properly treat the Donnan equilibrium established for charged species thereby potentially leading to erroneous results with charged polymers. This method of extending the pressure range is not needed in the operation of the present invention as the reference solution is simply dialyzate.
The present invention capitalizes upon advances in stirred cell dialysis chamber technology to improve osmometer design. Stirred cell dialysis chambers are available from, for example, Amicon (Beverly, Mass.). The principal use of the dialysis chamber in the prior art is to either concentrate a sample solution or to remove small molecule impurities by exhaustive flushing with pure solvent. The stirred cell is designed to be dismantled easily for cleaning. It has a magnetic stirring rod suspended above the membrane to keep the solution well stirred and to sweep clean the surface of the membrane. It can withstand more than 75 psi of external pressure, far exceeding the pressure measurable in conventional membrane osmometers (.about.1-3 psi).